Silica gels have been manufactured for many years and utilized in a plethora of different applications, ranging from abrasives to desiccants to thickeners and the like. The typical manufacturing process followed for silica gel production involves careful feeding of an acid and water glass solution through a narrow tube into a large cylindrical vessel. The gel will form nearly instantaneously so it is imperative that the feed be undertaken in such a fashion as to permit the resultant highly viscous gel product to form from the bottom of the cylinder and fill up the entire structure without clogging the feed system itself. Upon complete gel formation, the cylinder is then tipped downward and a piston is triggered to force the gel (in a large cylindrically shaped semi-solid form) out of the cylinder and through a series of mesh screens of differing gauge in order to “slice” the large gel mass into discrete cubic (or chunks) shapes. The resultant small gel cubes are then ammoniated and subjected to heating for a sufficient time to provide a desired pore size level and reinforcement simultaneously. The dried particles can then be milled to a desired size.
Classic silica gel manufacturing methods, involving the same general instantaneous gel production as discussed above, are disclosed within U.S. Pat. Nos. 3,794,712 and 3,819,811, to Aboutboul et al., as well as U.S. Pat. No. 4,148,864 to Groth et al. In more detail, hydrous silica gels are the result of the classical reaction of an alkali silicate with a mineral acid. Sulfuric acid is the most commonly used acid, although other mineral acids such as hydrochloric acid, nitric acid or phosphoric acid can be used. Sodium or potassium silicate may be used as the alkali silicate, with sodium silicate being preferred. The acid is added to the alkali silicate solution until a pH of less than about 5 is reached, with a pH of about 3 to 4.5 being most common. The alkali silicate solution can be mixed during this addition. The resulting product is a solid silica which includes the liquid phase. That is, the silica fully includes the water within its pores. For this reason that the solid phase contains the liquid phase, these silica materials have been termed silica hydrogels, with the dried silica being termed a silica gel. The mode of drying will determine whether the silica gel is a silica aerogel or a silica xerogel. Such silica hydrogels after synthesis have a water content of about 50 to 85 percent by weight.
Traditionally, in producing these silica hydrogels, the alkali silicate solution has an SiO2 concentration of about 6 to 30 percent by weight. A stoichiometric excess of acid is used, thereby reaching the low preferred pH of 3 to 4.5. After the silica hydrogel is formed, it is washed with ammonia to remove excess salts and then dried (by any standard manner, such as oven drying, spray drying, flash drying, and the like) for a specified period of time to target pore sizes and pore volumes therein.
Due to the instantaneous gel formation effectuated within typical gel manufacturing equipment, there is no possibility for the introduction of extra caustic agent within the typical gel reaction process. If such an addition were made, the mineral acid reactant would be neutralized and no appreciable gel formation would occur. Conventionally, subsequent to the gel formation, the cylindrical mass is sliced into individual gel units; so gel unit would be modified to an appreciable degree upon exposure to a caustic material either. Thus, there has been no teaching of modification during gel formation steps previously.
As with other silica products, the generation of a specific pore size and particle size is the frequent goal of silica gel materials manufacture. Gels can be targeted for any such physical characteristic depending on the manufacturing method employed, in particular, as it pertains to the aforementioned traditional gel manufacturing method, the longer the aging time at a certain temperature will cause a certain level of pore size and pore volume to result. In general, the following of this aforementioned typical gel manufacturing procedure entails a large amount of heat over a long period of time in order to target the desired pore size. As such, the methods followed over the years have proven to be rather inefficient and costly. Likewise, the machinery and instrumentation required for such gel manufacture are, as the description above indicates, rather complex in nature and bulky, not to mention potentially prone to mechanical difficulties. Hence, a less complex method of manufacture with typical silica production apparatuses would be a preferable development for the silica gel manufacturer as well. Furthermore, since caustic additions have heretofore been nonexistent for gel formation methods, the introduction of any caustic materials that could impart a metal species to the gel surface to form a composite gel has not been disclosed within the prior art either. A method that enhances such composite formation (such as with desirable alkaline earth metal species) would thus be desired within the silica gel industry. Yet, to date, no such efficient and reliable process has been forthcoming.